1. Field of the Invention
The present invention relates to a negatively chargeable developing agent for mono-component development, mono-component developing device using the developing agent and image-forming apparatus, being suitable for electrophotography, electrostatic recording, and electrostatic printing.
2. Description of the Related Art
For development of electrostatic latent images there have been known two systems, one known as two component development system wherein a mixture of a magnetic carrier and a non-magnetic toner is used, the other known as mono-component development system wherein no carrier is used. The two component development system has occupied a major share and has therefore been more widely used. Recently, however, the mono-component development system has been widely used in the art, because the system does not require the use of carrier and, hence, does not require carrier replacement, and because stable image-formation can be achieved by using a developing unit which is compact and simple in construction.
The mono-component development system is a system such that a thin layer of charged toner, formed on a developing sleeve as toner particles are forced to pass through a pressure contact clearance between the sleeve and a toner regulating blade, acts to develop an electrostatic latent image formed on a photosensitive member. Toner charging is effected at the pressure contact portion of the toner regulating blade.
However, the time allowed for the toner to contact the toner regulating blade is so short that toner particles should be triboelectrically charged to a predetermined charge level within an extremely limited period of time. Therefore, it is required that the toner be able to be charged quickly to the predetermined charge level.
If the toner charge is insufficient or if the toner is excessively charged, with the result that there should occur fluctuations in the quantity of charge, it is not possible to achieve any smooth and uniform image transfer from the development sleeve to the photosensitive member and/or from the photosensitive member to a recording medium, such as paper. This may cause fogging with respect to images formed.
Toner particles left on the developing sleeve after development of the electrostatic latent image on the photosensitive member returns to the charging region between the regulating blade and the developing sleeve so that the remaining toner particles go into contact with the regulating blade for being charged again. This means that the toner particles are triboelectrically charged two times. As a result, the toner is electrically charged more than normally required. The presence of such toner particles causes variations in toner charge, and this inevitably results in aforementioned fogging. Therefore, once the toner is charged to a specified level, stability at that level is required of the toner.
Toner charge varies according to the environmental condition in which the toner is placed. Therefore, it is required that toner charge should have good environmental stability.